DC Power Supply Design

Discussion in 'General Electronics Chat' started by DC_Kid, Feb 19, 2015.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    i havent been here in quite some time, looking for some design advice.

    i am building a nifty landscape lighting supply (four 12vdc output channels, mosfet controlled outputs, with PWM driver in the mix). each channel will be rated/regulated 12vdc @ 5amp.

    my questions:

    1) to get good regulation the DC(in) to reg should be ~15v possibly more. thus a std 12vac(out) xfrmer wont work, need something like a 15vac or higher xfrmer? i can get some decent inexpensive 24vac/5amp xfrmers. would 24vac xfrmer be ok for most linear regs?
    2) is it more efficient to build full wave rectifier with two diodes and CT xfrmer, or non CT xfrmer and four diodes?
    3) its cheaper to get four 5amp xfrmers than one 20amp item. should i build four discrete power supplies in one case, or can i parallel the xfrmers (phased together) and just build one large power supply? i would use a hefty bridge rectifier but i would then use individual regs for each channel.

  2. #12


    Nov 30, 2010
    A 12 volt transformer:
    (12 times sqrt 2) minus 1.4V for the diodes minus 3V for the regulator chip = 12.57 volts.
    That leaves 0.57 volts of p-p ripple on the filter capacitor.
    A single winding with 4 diodes is the best use of the transformer because all the turns are working on every half cycle. You can only get 62% of the rated current out of that configuration. 5amps/.62 = 8.06 amp rating needed.
    You can parallel the windings if the transformers are a matched set and you pay attention to get the phasing right.
    My preference is to build 4 separate supplies so the whole setup doesn't go down for a single failure, and it's easier to get rid of the waste heat.
    Using a low dropout regulator chip will allow the filter capacitors to be smaller.
    Building only one supply is cheaper, I think.
    You are free to use something different.

    One thought is a pile of (4) 60 watt switching supplies which might be bought cheaply and quickly.
    Last edited: Feb 19, 2015
    planeguy67 likes this.
  3. MrChips


    Oct 2, 2009
    #12 and planeguy67 like this.
  4. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    An ATX psu will give 22Amps @ 12V easy.....cheaper
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    I like the number of neophytes that come here wanting to get hands on and build stuff they could buy for just a small sum of money. It gives me hope for our engineering future. :)
    absf likes this.
  7. jgessling

    Well-Known Member

    Jul 31, 2009
  8. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    i would put the rail into a metal enclosure which is being mounted outside (metal chassis with clear plastic front will suffice given my mounting location) . since this is for landscape lighting each unit line in would be controlled via a 4 channel timing unit that controls a relay to power each PS. i will build a mosfet stage to handle the 12vdc channels so i can connect some already made PWM controllers.

    post #6. whaaaaat, its why people come here, to learn and build even if its cheaper to buy joe-doodle's item off of ebay, etc. in this case though, the PS i listed will save me $$ and time.